Notes from soggy bottom

Monday, July 04, 2011

Downhome Breeding Birds Plus 5 Years

Having completed last year my little project of running BBS-style surveys on nearly every public road in the county, this year it was time to start my 5-years-later resample. I reran the first three routes, run previously in 2006. Putting the data side-by-side, they suggest some substantial changes over that time; but three routes of 50 stops each is not a big sample. Still, it is the minimum necessary to do a paired t-test on them and look for any statistically significant changes. So I did just that.

The particulars -- square-root transformed data, N = 3, DF = 2, total species examined = 86. Note that at a significance level of p=.05 I would expect 4 or 5 spurious hits; at p=.01 I would expect about 1 spurious hit.

A total of 5 species showed a significant change (p=.05) from 2006 to 2011. Those marked with ** are also significant at the p=.01 level (2 species)

The power of this test is very low; many species showed large differences between the two years that were not statistically significant. This is not a whole lot more than what would be expected from random data, except for one thing: ALL of these changes are increases. No species showed a significant decline.

The biggest (statistically) change was the increase in Scarlet Tanagers. This is consistent with my general impression that this bird has substantially increased in middle Tennessee since I arrived in 2002. It does not seem to have been at the expense of Summer Tanagers, however: their total for these three routes were exactly the same in 2006 and 2011.

A lot of species showed pretty large changes that were not significant using this (low power) test. Many of these changes were consistent with patterns I feel like I have been seeing in general -- for example, increases in all woodpeckers except Northern Flicker, which is decreasing. When I get the chance I hope to pull up the statewaide long-term BBS data for Tennessee and see what it shows for the larger trends.